1. Field of the Invention
The present invention relates to an automatic focusing apparatus which can be applied to an auto-level, a theodolite or a transit instrument etc., having a telephotographic system.
2. Description of the Related Art
A surveying instrument, such as an auto-level, a theodolite or a transit instrument etc., is basically provided with a collimating telescope, a level, and scales for measuring a rotative angle or an elevational angle. The surveying instrument is set in a horizontal plane and is adjusted in horizontal and vertical directions so that an operator may observe a collimation point or object through a collimating telescope.
A typical surveying instrument having a collimating telescope is provided, in an order from an object side, with an objective lens, a focusing lens and an eyepiece. The position of the focusing lens is adjusted according to the distance from the object, so that an image of the object may be formed on a focusing plate (reticle). The operator may thus observe the image superimposed on the reticle via the eyepiece.
For example, provided that the scope range of the object distance is between 1 m--infinity (this range is very wide compared with a telescope or binoculars), and that the focusing lens is a concave lens, the approximate amount of movement of the focusing lens is 30 mm. The focusing lens is normally driven through an operation of a rotative knob. If the amount of movement of the object image, namely the amount of movement of the focusing lens, is set smaller in regard to the rotative angle of the rotative knob, the amount of movement of the object image becomes smaller as compared with the rotative angle. Therefore, a relatively large amount of time is required to move such a lens. On the other hand, if the amount of movement of the object image is set larger in regard to the rotative angle of the rotative knob, the amount of movement of the object image becomes great compared with the rotative angle. Such an arrangement leads to difficulty in stopping the object image on the reticle.
Additionally, if the object is positioned far in the distance, the object image moves a large amount forwardly and rearwardly through a small operation of the rotative knob in the rotative direction. On the other hand, if the object is positioned relatively close to the focusing lens, the amount of movement of the object image becomes smaller as compared with the amount of rotation of the rotative knob, thus it is required to move the rotative knob a great deal for the purpose of moving the object image on the reticle. In such a case, it is difficult to determine whether the aiming object is in a front focus or a rear focus position, which may lead the operator to rotate the rotative knob in the wrong direction opposite to the focus direction. In any event, a conventional auto-level suffers from the fact that it takes too much time for focusing.
In order to overcome the above mentioned problem, a survey instrument is provided in which a so-called passive type of automatic focusing apparatus is mounted. As is well known, the focusing of the automatic focusing apparatus is performed, firstly by dividing an optical path of the optical system of the collimating telescope, and secondly by detecting a defocus along the divided optical path, based on the object image focused at the same position as that of the focusing plate.
However, the survey instrument normally utilizes a staff which serves as the aiming object, so that the staff is positioned at the center of the visual field (on the vertical line of the reticle). The size of the staff image in the visual field becomes smaller especially as the distance becomes longer. Consequently, the conventional auto-level may focus on the background of the staff in the case of high contrast, namely when the staff is dark while the background behind the staff is brighter.